In a high specific speed centrifugal compressor with a vaneless diffuser, there is significant nonuniformity of the flow at the impeller exit in the passage defined by the vaneless diffusers. For radial discharge impellers, where the flow leaves the impeller 90.degree. to the impeller axis in the meridional plane, this nonuniformity is generally characterized by a large region of low radial velocity and low angle flow, relative to the tangential, near the shroud side of the passage. There can also be a small amount of nonuniformity near the hub side. For mixed flow impellers, where the flow leaves the impeller at less than 90.degree. to the impeller axis in the meridional plane, there is generally a redistribution of the nonuniformity in the direction of reducing the severity and extent of the nonuniformity near the shroud side of the passage and increasing the nonuniformity near the hub side.
There are several penalties associated with these flow nonuniformities if they are allowed to go unchecked. A low radial velocity fluid near the side walls of the vaneless diffuser will quickly be brought to rest and then pushed in towards the impeller by the increasing pressures within the diffuser. Thus, part of the flow is pushed back into the impeller where it has to be reprocessed. This leads to increased impeller work and decreased stage efficiencies. In addition, this reverse flow effectively blocks the passage in the vaneless diffuser, so that flow diffusion is diminished and efficiency suffers. The continued nonuniformity in the vaneless diffuser reduces the effectiveness and efficiency of the vaneless diffuser and downstream components.
The purpose of a rib diffuser is to correct the nonuniformity at the impeller tip by placing stationary ribs as near as practical to the impeller tip and only in regions having sufficiently low angle flow. This configuration avoids the problems associated with full vane diffusers which have vanes in the parts of the flow not requiring correction thus creating frictional and wake losses. Existing ideas attempt to correct the nonuniformity at the impeller tip by fixing a single row of ribs on the diffuser wall that extends across the complete region of low angle flow. The region of nonuniformity can be a significant percentage of the passage width at the impeller tip, and the flow angles can vary significantly over this region, up to 20.degree.-30.degree.. Consequently, a single row of ribs set at some average angle will see a large flow incidence variation, .+-.10.degree.-15.degree. or even .+-.20.degree., over the vane leading edge height. Incidence levels could be large enough to cause leading edge flow separation and/or significant boundary layer growth leading to flow losses and lower efficiencies.
Another existing idea attempts to correct nonuniformity of flow with the combination of a single row of ribs, a full vane and fins or grooves on the impeller. Because the full vane extends through areas of uniform flow, frictional and wake losses are incurred.
Thus, a need exists to more accurately correct and redirect the nonuniform flow regions without large flow incidence variations associated with single row rib diffusers and without additional frictional or wake losses associated with full vane diffusers.